1. Field of the Invention
This invention relates to a valve driving device for an internal combustion engine, and more particularly to a type thereof including a hydraulic lifter in each of the rocker arms.
2. Discussion of the Background
In an ordinary internal combustion engine, it has been required to provide a valve clearance of a predetermined length for eliminating adverse effects caused by differences between the thermal expansions of the cylinder head, cylinder block and the like and those of the valve driving mechanism. In the situation where the clearance is not maintained correctly, noises and/or output loss due to the leakage of crude gas tend to occur during the operation. For preventing such a drawback, a hydraulic lifter has been proposed which maintains the valve clearance at zero during the operation of the valve driving mechanism while eliminating the noise and the output loss and stabilizing the operation of the mechanism. The present invention relates to a valve driving device for an automobile engine in which the hydraulic lifters are utilized.
Japanese Patent Disclosure No. 16,113/1978 discloses such a valve driving device for an internal combustion engine including hydraulic lifters. In this device, each hydraulic lifter is encased in a rocker arm, one end thereof receiving a load from a cam shaft while the other end thereof driving a valve stem. Each hydraulic lifter comprises a plunger slidably encased in the rocker arm so that one end of the plunger operates the valve stem, while the other end thereof forms a pressurized chamber, a reservoir formed in the body of the rocker arm, and a check valve which permits flow of a pressurized oil only in a direction from the reservoir to the pressurized chamber. An oil passage is further provided through a rocker shaft swingably supporting the rocker arm, so that an operative oil such as engine oil supplied from an oil pump is sent through the oil passage into the reservoir of the hydraulic lifter.
With the above described valve driving device, however, a large amount of bubbles tend to be mixed in the operative oil supplied into the reservoir through the oil passage formed in the rocker shaft at the time of engine start or abruptly increasing or decreasing the rotating speed of the engine. The bubbles are collected in an upper part of the reservoir, and when the amount thereof becomes excessive, the bubbles intrude into the pressurized chamber, losing the rigidity of the hydraulic lifter and entailing creation of impacting sound between the plunger and its stopper or between the valve and the valve seat.